1. Field of the Invention
The present invention relates to an electrical discharge machine which applies discharge pulses of both polarities to an electrode and a workpiece to machine the workpiece by means of a discharge in electrical discharge machining.
2. Description of the Background Art
Conventionally, since a dielectric fluid made of water and a compound or oil is used in diesinking electrical discharge machining, the tar decomposition of the compound or oil builds up on the machined surface of a workpiece particularly finishing, posing problems of unstable machining, poor machined surface roughness, etc. To solve these problems, there exists a method of temporarily reversing the polarities of the electrode and the workpiece during machining as disclosed in Japanese Laid-Open Patent Publication No. SHO59-93228, Japanese Laid-Open Patent Publication No. HEI3-196916, Japanese Laid-Open Patent Publication No. HEI3-239413, Japanese Laid-Open Patent Publication No. HEI4-101722, etc. Also, an example of a conventional electrical discharge machine using pulses of both polarities is disclosed in Japanese Laid-Open Patent Publication No. HEI3-208520, etc. Further, the control of pulses of both polarities in electrical discharge machining using the pulses of both polarities is described in Japanese Laid-Open Patent Publication No. SHO61-50714.
The methods disclosed in Japanese Laid-Open Patent Publication No. SHO59-93228, Japanese Laid-Open Patent Publication No. HEI3-196916 and Japanese Laid-Open Patent Publication No. HEI3-239413 allow built-up tar to be removed by an opposite polarity discharge. However, the methods disclosed in these publications, wherein voltages at positive polarity (negative electrode, positive workpiece) and opposite polarity (positive electrode, negative workpiece) are judged at proper intervals, have problems in that machining at positive polarity increases electrode consumption generally and machining speed is effected because the machining status is extremely unstable for a while after the reversal of the polarity due to a difference in machining status between the positive and negative polarities.
The method disclosed in Japanese Laid-Open Patent Publication No. HEI4-101722 allows machining parameters to be set separately for positive-polarity machining and opposite-polarity in order to reduce electrode consumption to a certain degree, but the method is identical to those disclosed in Japanese Laid-Open Patent Publication No. SHO59-93228, Japanese Laid-Open Patent Publication No. HEI3-196916 and Japanese Laid-Open Patent Publication No. HEI3-239413 in that a workpiece is machined at two polarities, and cannot substantially reduce electrode consumption. Also, if positive polarity machining conditions are lowered to reduce electrode consumption, the machining speed of the workpiece is inevitably reduced during positive polarity machining. Further, especially if the machining current value is decreased (as one of the machining conditions that is lowered), the use of a water-based dielectric fluid has a strong possibility of causing an extremely unstable status which will not raise a voltage and will not generate a proper discharge.
An example of the electrical discharge machine using pulses of both polarities to prevent the deterioration of face roughness due to electrolytic phenomenon is disclosed by Japanese Laid-Open Patent Publication No. HEI3-208520. It is recognized therein that there is a phenomenon wherein the roughness of the machined surface when the electrode is positively energized is made 1.5 to 2.0 times rougher than when the electrode is negatively energized, and hence the total roughness of the surface becomes rougher than that which is obtained when the electrode is positively energized. In order to eliminate these problems, which still remain when pulses of both polarities are used, the voltage of one polarity is set to a value at which discharge occurs, whereas the voltage of opposite polarity is set to a value at which no discharge occurs. However, in order to set the voltage of opposite polarity to a value at which no discharge occurs so that the average voltage between the electrodes drops to 0 or close to 0, it is necessary to increase the period of time during which the voltage of opposite polarity is applied to the electrode; that is, the period of time necessary for applying the voltage of opposite polarity is made longer. As a result, a problem arises because the machining speed is also lowered. Since the electrical discharge machine disclosed in Japanese Laid-Open Patent Publication No. HEI3-208520 is intended to prevent an electrolytic action, a leakage current flowing in the machining gap through the dielectric fluid may be nearly zero on average. However, this system also not adequate for the prevention of tar build-up because the phenomenon of charged tar being attracted by the voltage of the electrode or workpiece creates a problem.
Furthermore, Japanese Laid-Open Patent Publication No. SHO61-50714 describes the control of pulses of both polarities in electrical discharge machining. This is a method of applying an opposite polarity voltage for a very short time when a discharge is not generated by the application of a pulsed voltage. This publication also describes a method of increasing the voltage and time for applying a low-level pulse of the opposite polarity voltage when the discharge does not take place. However, merely applying a pulse for a very short time cannot drop the average voltage sufficiently and the average voltage of 18 V provided by this machine has almost no effect on the prevention of tar build-up. Since this machine is also intended to prevent an electrolytic action, a leakage current flowing in the machining gap through a dielectric fluid may be nearly zero on average. However, this system is not adequate for the prevention of tar build-up because the charged tar is attracted by the voltage of the electrode or workpiece. Also, the method of increasing the voltage and time of a low-level pulse of opposite polarity voltage applied when the discharge does not take place is useless because the non-occurrence of a discharge is totally irrelevant to the stability or instability of a discharge status. Since a discharge does not occur mainly because the electrode and workpiece are away from each other, the detection of the discharge status to control machining conditions does not produce an effect on the improvement of machining performance.
To summarize diesinking electrical discharge machine generally employs oil or a mixture of water and a high molecular compound as a dielectric fluid. When this dielectric fluid is used for electrical discharge machining, the oil or compound is decomposed to generate tar. This tar, which has been charged, builds up on the electrode in the dielectric fluid of the oil or on the workpiece in the dielectric fluid made of the mixture of water and a high molecular compound, posing problems of unstable machining and poor machined surface roughness. To prevent the charged tar from building up on the electrode or the workpiece, the workpiece was conventionally machined with a voltage of opposite polarity. However, this method presented a machining speed problem and also a problem of increased electrode consumption because low electrode consumption conditions cannot be maintained if the polarity is switched for machining. Also, conventionally there were no measures for the detection method, machining condition control, etc., of unstable machining which was the cause of tar build-up.
The of the inventors revealed the following three facts. First, controlling the average voltage of the machining gap to nearly 0 V is effective for preventing the charged tar from building up on the electrode or workpiece. Second, surface quality is enhanced not only by making the average voltage 0 V, but also by causing a slight discharge at opposite polarity. It should be noted that the machining current must be reduced for machining at positive polarity (negative electrode, positive workpiece) because of its high electrode consumption rate. Third, when machining has become unstable due to tar building up on the electrode or workpiece, rendering the average voltage of the machining gap to 0 V returns the machining to a stable status immediately in as short as several seconds. To increase the machining speed, therefore, it is effective to increase the stop time, i.e., the time when a positive polarity voltage is applied, to cause the average voltage of the machining gap to approach 0 V on detection of instability while the workpiece is being machined with the duty factor of the opposite polarity increased.